Internal combustion engine

ABSTRACT

A fuel injector and a spark plug are arranged inside a combustion chamber of an internal combustion chamber. Fuel is injected from the fuel injector at the end of the compression stroke. When the amount of EGR gas is increased, the amount of production of soot peaks. When the amount of EGR gas is further increased, soot is no longer produced. The amount of EGR gas in the combustion chamber is made larger than the amount of EGR gas where the amount of production of soot peaks. Due to this, the production of soot in the combustion chamber is suppressed.

TECHNICAL FIELD

The present invention relates to an internal combustion engine.

BACKGROUND ART

Known is a direct injection type internal combustion engine which formsan air-fuel mixture in a limited region of a combustion chamber andignites the air-fuel mixture by a spark plug when the engine load isrelatively low and which fills the combustion chamber with a uniformair-fuel mixture and ignites the uniform air-fuel mixture by a sparkplug when the engine load becomes higher. In this direct injection typeinternal combustion engine, normally, for example as disclosed inJapanese Unexamined Patent Publication (Kokai) No. 5-18245, the sparkplug is arranged at the center of the inner wall surface of a cylinderhead, a groove extending from below a fuel injector to below the sparkplug is formed in a top surface of a piston, fuel is injected toward thegroove when the engine load is relatively low, and the injected fuel isguided by the bottom surface of the groove to form an air-fuel mixturein a limited region around the spark plug.

If fuel is injected from the fuel injector, however, right afterinjection, an overly rich air-fuel mixture is formed at the center ofthe fuel mist. Therefore, if the air-fuel mixture is ignited by thespark plug right after fuel injection, the overly rich air-fuel mixtureis burned and as a result a large amount of soot is produced.Accordingly, in the past, in direct injection type internal combustionengines, the practice had been to advance the fuel injection timing tocause the injected fuel to disperse before ignition and eliminate thepresence of an overly rich air-fuel mixture region around when themixture was ignited and thereby prevent the generation of soot.

When forming an air-fuel mixture in a limited region in a combustionchamber, however, if advancing the fuel injection timing to cause theinjected fuel to disperse in this way, a considerably lean air-fuelratio region is formed over an extensive area around the air-fuelmixture. If a considerably lean air-fuel ratio region is formed over anextensive area in this way, however, the flame of ignition of the sparkplug will not be propagated well in that region and therefore a largeamount of unburned hydrocarbons will be produced. That is, the amount offuel not being burned well will increase, so the problem of an increasein the amount of fuel consumption will arise.

In this case, if delaying the fuel injection timing to ignite theair-fuel mixture before the injected fuel disperses, the flame ofignition will quickly be propagated to the air-fuel mixture as a wholeand the air-fuel mixture as a whole will be burned. As a result, almostno unburned hydrocarbons will be produced and the amount of fuelconsumption can be reduced. At this time, however, an overly richair-fuel mixture region will be formed, so as explained above a largeamount of soot will be produced.

If a large amount of soot were not produced at this time, no unburnedhydrocarbons would be produced and ideal combustion with little fuelconsumption could be obtained.

On the other hand, in the past, in internal combustion engines, theproduction of NO_(x) has been suppressed by connecting the engineexhaust passage and the engine intake passage by an exhaust gasrecirculation (EGR) passage so as-to cause the exhaust gas, that is, theEGR gas, to recirculate in the engine intake passage through the EGRpassage. In this case, the EGR gas has a relatively high specific heatand therefore can absorb a large amount oft heat, so the larger theamount of EGR gas, that is, the higher the EGR rate (amount of EGRgas/(amount of EGR gas+amount of intake air), the lower the combustiontemperature in the engine intake passage. When the combustiontemperature falls, the amount of NO_(x). produced falls and thereforethe higher the EGR rate, the lower the amount of NO_(x) produced.

In this way, in the past, it was known that the higher the EGR rate, thelower the amount of NO_(x) produced can become. If the EGR rate isincreased, however, the amount of soot produced, that is, the smoke,starts to sharply rise when the EGR rate passes a certain limit. In thispoint, in the past, it was believed that if the EGR rate was increasedfurther, the concentration of oxygen. around the fuel would fall andresult in an overly rich mixture and the smoke would increase withoutlimit. Therefore, it was believed that the EGR rate at which smokestarts to rise sharply was the maximum allowable limit of the EGR rate.

Therefore, in the past, the EGR rate was set within a range notexceeding this maximum allowable limit. The maximum allowable limit ofthe EGR rate differed considerably according to the type of the engineand the fuel, but was from 30 percent to 50 percent or so. Accordingly,in conventional internal combustion engines, the EGR rate was suppressedto 30 percent to 50 percent or so at a maximum.

Since it was believed in the past that there was a maximum allowablelimit to the EGR rate, in the past the EGR rate had been set within arange not exceeding that maximum allowable limit so that the amount ofNO_(x) produced would become as small as possible. Even if the EGR rateis set in this way so that the amount of NO_(x) produced becomes assmall as possible, however, there are limits to the reduction of theamount of production of NO_(x). In practice, therefore, a considerableamount of NO_(x) continues being produced.

In the process of studying the combustion in internal combustionengines, however, the present inventors discovered that if the EGR rateis made larger than the maximum allowable limit, the smoke sharplyincreases as explained above, but there is a peak to the amount of thesmoke produced and once this peak is passed, if the EGR rate is madefurther larger, the smoke starts to sharply decrease and that if the EGRrate is made at least 70 percent during engine idling or if the EGR gasis force cooled and the EGR rate is made at least 55 percent or so, thesmoke will almost completely disappear, that is, almost no soot will beproduced. Further, they found that the amount of NO_(x) produced at thistime was extremely small. They engaged in further studies later based onthis discovery to determine the reasons why soot was not produced and asa result constructed a new system of combustion able to simultaneouslyreduce the soot and NO_(x) more than ever before. This new system ofcombustion will be explained in detail later, but briefly it is based onthe idea of stopping the growth of hydrocarbons into soot at anintermediate stage before the hydrocarbons grow.

That is, what was found from repeated experiments and research was thatthe growth of hydrocarbons stops at an intermediate stage beforebecoming soot when the temperature of the fuel and the gas around thefuel at the time of combustion in the combustion chamber is lower than acertain temperature and the hydrocarbons grow to soot all at once whenthe temperature of the fuel and the gas around the fuel becomes higherthan a certain temperature. In this case, the temperature of the fueland the gas around the fuel is greatly affected by the heat absorbingaction of the gas around the fuel at the time of combustion of the fuel.By adjusting the amount of heat absorbed by the gas around the fuel inaccordance with the amount of heat generated at the time of combustionof the fuel, it is possible to control the temperature of the fuel andthe gas around the fuel.

Therefore, if the temperature of the fuel and the gas around the fuel atthe time of combustion in the combustion chamber is suppressed to nomore than a temperature at which the growth of the hydrocarbons stopsmidway, soot is no longer produced. The temperature of the fuel and thegas around the fuel at the time of combustion in the combustion chambercan be suppressed to no more than a temperature at which the growth ofthe hydrocarbons stops midway by adjusting the amount of heat absorbedby the gas around the fuel. On the other hand, the hydrocarbons stoppedin growth midway before becoming soot, that is, the unburnedhydrocarbons, are exhausted, but the amount of exhaust of the unburnedhydrocarbons is far less than when advancing the fuel injection timing.

Therefore, when delaying the fuel injection timing, if using this newcombustion system, even if an overly rich air-fuel mixture region isformed, almost no soot is produced and further almost no NO_(x) isproduced. Therefore, ideal combustion in which almost no soot and NO_(x)are produced and the amount of fuel consumption is small can beobtained.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide an internal combustionengine capable of obtaining this ideal combustion.

According to the present invention, there is provided an internalcombustion engine provided with a spark plug for igniting fuel injectedinto a combustion chamber and in which an amount of production of sootpeaks when an amount of inert gas in the combustion chamber is increasedif delaying a fuel injection timing in a compression stroke, wherein theamount of inert gas in the combustion chamber is made greater than theamount of insert gas at which the amount of production of soot peaks andthereby the temperature of the fuel and the gas around it at the time ofcombustion in the combustion chamber is suppressed to a temperaturelower than the temperature at which soot is produced, whereby theproduction of soot is suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall view of a spark ignition type internal combustionengine;

FIG. 2 is a side sectional view of the internal combustion engine shownin FIG. 1;

FIG. 3 is a bottom view of a cylinder head shown in FIG. 2;

FIG. 4 is a view of the amount of generation of smoke, hydrocarbons, andNO_(x), etc.;

FIG. 5 is a view of the relationship between the amount of generation ofsmoke and the EGR rate;

FIG. 6 is a view of the relationship between the amount of injected fueland the EGR rate;

FIG. 7 is a view of a first operating region I, a second operatingregion II, and a third operating region III;

FIG. 8 is a view of an opening degree of a throttle valve etc.;

FIG. 9 is a view of an injection timing and ignition timing etc.;

FIG. 10 is a view of an air-flow ratio in the first operating region I;

FIGS. 11A and 11B are maps of the target opening degrees of the throttlevalve etc.;

FIG. 12 is a view of the air-flow ratio in second combustion;

FIGS. 13A and 13B are views of maps of the target opening degrees of athrottle valve etc.;

FIG. 14 is a flow chart of the control of the engine operation;

FIGS. 15A and 15B are views for explaining the action of absorption andrelease of NO_(x);

FIG. 16A, FIG. 16B, and FIG. 16C are maps of the amount of absorptionand release of NO_(x);

FIG. 17 is a flow chart of the processing of an NO_(x) release flag;

FIG. 18 is a flow chart of another embodiment of control of the engineoperation;

FIG. 19 is a view of the injection timing and ignition timing in anotherembodiment;

FIG. 20 is an overall view of another embodiment of a spark ignitiontype internal combustion engine; and

FIG. 21 is a view of the injection timng and ignition timing etc.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 to FIG. 3 show the case of application of the present inventionto a four-stroke spark ignition type internal combustion engine.

Referring to FIG. 1 to FIG. 3, 1 shows an engine body, 2 a cylinderblock, 3 a cylinder head, 4 a piston, 5 a combustion chamber, 6 anelectrically controlled fuel injector, 7 a spark plug, 8 a pair ofintake valves, 9 an intake port, 10 a pair of exhaust valves, and 11 anexhaust port. As shown in FIG. 2 and FIG. 3, the fuel injector 6 and thespark plug 7 are arranged adjoining each other at the center of theinner wall surface of the cylinder head 3.

As shown in FIG. 1, the intake port 9 is connected through acorresponding intake pipe 12 to a surge tank 13. The surge tank 13 isconnected through an intake duct 14 to an air cleaner 15. Inside the airduct 14 is arranged a throttle valve 17 driven by a step motor 16. Onthe other hand, the exhaust port 11 is connected through an exhaustmanifold 18 and exhaust tube 19 to a catalytic converter 21 housing acatalyst 20 having an oxidation function. Inside the exhaust manifold 18is arranged an air-fuel ratio sensor 22.

The exhaust manifold 18 and the surge tank 13 are connected with eachother through an EGR passage 23. Inside the EGR passage 23 is arrangedan electrically controlled EGR control valve 24. Further, inside the EGRpassage 23 is arranged an intercooler 25 for cooling the EGR gas flowingthrough the EGR passage 23. In the embodiment shown in FIG. 1, enginecooling water is led into the intercooler 25 and that engine coolingwater used to cool the EGR gas.

On the other hand, each fuel injector 6 is connected through a fuelsupply line 26 to a fuel reservoir 27. Fuel is supplied to the fuelreservoir 27 from an electrically controlled variable discharge fuelpump 28. Fuel supplied in the fuel reservoir 27 is supplied through thefuel supply lines 26 to the fuel injectors 6. A fuel pressure sensor 29for detecting the fuel pressure in the fuel reservoir 27 is attached tothe fuel reservoir 27. The amount of discharge of the fuel pump 28 iscontrolled based on the output signal of the fuel pressure sensor 29 sothat the fuel pressure in the fuel reservoir 27 becomes the target fuelpressure.

An electronic control unit 30 is comprised of a digital computer and isprovided with a ROM (read only memory) 32, a RAM (random access memory)33, a CPU (microprocessor) 34, an input port 35, and an output port 36connected with each other by a bidirectional bus 31. The output signalof the air-fuel ratio sensor 22 is input through a corresponding ADconverter 37 to the input port 35, while the output signal of the fuelpressure sensor 29 is also input through a corresponding AD converter 37to the input port 35. The accelerator pedal 40 has connected to it aload sensor 41 for generating an output voltage proportional to theamount of depression L of the accelerator pedal 40. The output voltageof the load sensor 41 is input through a corresponding AD converter 37to the input port 35. Further, the input port 35 has connected to it acrank angle sensor 42 for generating an output pulse each time thecrankshaft rotates by for example 30°. On the other hand, the outputport 36 has connected to it through a corresponding drive circuit 38 thefuel injectors 6, the step motor 16, the EGR control valve 24, and thefuel pump 28.

FIG. 4 shows an example of an experiment showing the changes in theoutput torque and the amount of exhaust of smoke, hydrocarbons, carbonmonoxide, and NO_(x) when changing the air fuel ratio A/F (abscissa inFIG. 4) by changing the opening degree of the throttle valve 17, the EGRrate, and the fuel injection timing when the engine load is relativelylow. As will be understood from FIG. 4, in this experiment, the EGR ratebecomes larger the smaller the air fuel ratio A/F. When the air-fuelratio is not more than 20, the EGR rate becomes more than 65 percent.Note that in FIG. 4, 20°, 40°, and 80° show the fuel injection timingsexpressed by values before top dead center of the compression stroke.

As shown in FIG. 4, if increasing the EGR rate to reduce the air fuelratio A/F, regardless of the fuel injection timing, the amount of NO_(x)produces gradually falls. On the other hand, when the fuel injectiontiming is early, that is, at BTDC 80°, regardless of the air-fuel ratioA/F, no smoke is generated, but a large amount of unburned hydrocarbonsis produced. That is, when the fuel injection timing is early, theinjected fuel is dispersed across an extensive area before being ignitedby the spark plug 7 and therefore there is no overly rich air-fuelmixture region and thus no production of soot. In this case, however, asmentioned at the start, a considerably lean air-fuel mixture region isformed, so a large amount of unburned hydrocarbons is produced.

As shown in FIG. 4, when the fuel injection timing is early, the amountof unburned hydrocarbons produced increases and the amount of fuelconsumption falls the greater the EGR rate. Therefore, it is notpreferable to increase the EGR rate when the fuel injection timing isearly.

As opposed to this, when delaying the fuel injection timing, forexample, when making the fuel injection timing BTDC 20°, an overly richair-fuel mixture region is formed at the time of the ignition action ofthe spark plug 7. In this case, as shown in FIG. 4, if the EGR rate isincreased, the amount of smoke produced starts to increase when the EGRrate reaches near 40 percent. Next, if the EGR rate is further raisedand the air-fuel ratio A/F reduced, the amount of smoke produced sharplyincreases and reaches a peak. Next, if the EGR rate is further raisedand the air-fuel ratio A/F reduced, the smoke then sharply falls. If theEGR rate is made more than 65 percent and the air-fuel ratio A/F becomesnot more than 20, almost zero smoke is produced. That is, almost no sootis produced any longer. At this time, the output torque of the enginefalls somewhat and the amount of production of NO_(x) becomesconsiderably low. On the other hand, the amounts of hydrocarbons andcarbon monoxide produced start to increase.

When the fuel injection timing is delayed, the following may be saidfrom the results of the experiment shown in FIG. 4. That is, first, whenthe air-fuel ratio A/F is not more than 20 and the amount of productionof smoke is substantially zero, as shown in FIG. 4, the amount of NO_(x)produced falls considerably. A fall in the amount of production ofNO_(x) means a fall in the combustion temperature in the combustionchamber 5. Therefore, it may be said that the combustion temperature inthe combustion chamber 5 becomes lower when almost no soot is produced.

Second, when the amount of smoke produced, that is, the amount of sootproduced, becomes substantially zero, as shown in FIG. 4, the amounts ofexhaust of hydrocarbons and carbon monoxide increase. This means thatthe hydrocarbons are exhausted without growing into soot. That is, thehydrocarbons contained in the fuel decompose when raised in temperaturein an oxygen poor state resulting in the formation of a precursor ofsoot. Next, soot mainly comprised of solid masses of carbon atoms isproduced. In this case, the actual process of production of soot iscomplicated. How the precursor of soot is formed is not clear, butwhatever the case, the hydrocarbons in the fuel grow to soot through thesoot precursor. Therefore, as explained above, when the amount ofproduction of soot becomes substantially zero, the amounts of exhaust ofhydrocarbons and carbon monoxide increase as shown in FIG. 4, but thehydrocarbons at this time are a soot precursor or a state ofhydrocarbons before that.

Summarizing these considerations based on the results of the experimentshown in FIG. 4, when the combustion temperature in the combustionchamber 5 is low, the amount of soot produced becomes substantiallyzero. At this time, a soot precursor or a state of hydrocarbons beforethat is exhausted from the combustion chamber 5. More detailedexperiments and studies were conducted on this. As a result, it waslearned that when the temperature of the fuel and the gas around thefuel in the combustion chamber 5 is below a certain temperature, theprocess of growth of soot stops midway, that is, no soot at all isproduced and that when the temperature of the fuel and its surroundingsin the combustion chamber 5 becomes higher than a certain temperature,soot is produced.

The temperature of the fuel and its surroundings when the process ofproduction of hydrocarbons stops in the state of the soot precursor,that is, the above certain temperature, changes depending on variousfactors such as the type of the fuel, the air-fuel ratio, and thecompression ratio, so it cannot be said what degree it is, but thiscertain temperature is deeply related with the amount of production ofNO_(x). Therefore, this certain temperature can be defined to a certaindegree from the amount of production of NO_(x). That is, the greater theEGR rate, the lower the temperature of the fuel and the gas surroundingit at the time of combustion and the lower the amount of NO_(x)produced. At this time, when the amount of NO_(x) produced becomesaround 10 ppm or less, almost no soot is produced any more. Therefore,the above certain temperature substantially matches the temperature whenthe amount of NO_(x) produced becomes 10 ppm or less. once soot isproduced, it is impossible to remove it by after-treatment using anoxidation catalyst etc. As opposed to this, a soot precursor or a stateof hydrocarbons before this can be easily removed by after-treatmentusing an oxidation catalyst etc. Considering after-treatment by anoxidation catalyst etc., there is an extremely great difference betweenwhether the

In this case, to suppress the temperature of the fuel and the gas aroundit to a temperature lower than the temperature at which soot isproduced, an amount of inert gas enough to absorb an amount of heatsufficient for this is required. Therefore, if the amount of fuelincreases, the amount of inert gas required increases along with thesame. Note that in this case, the larger the specific heat of the inertgas, the stronger the heat absorbing action. Therefore, the inert gas ispreferably a gas with a large specific heat. In this regard, since CO₂and EGR gas have relatively large specific heats, it may be said to bepreferable to use EGR gas as the inert gas.

FIG. 5 shows the relationship between the EGR rate and smoke when usingEGR gas as the inert gas and changing the degree of cooling of the EGRgas in the stae of a fuel injection timing of BTDC 20°. That is, thecurve A in FIG. 5 shows the case of force cooling the EGR gas andmaintaining the temperature of the EGR gas at about 90° C., curve Bshows the case of cooling the EGR gas by a compact cooling apparatus,and curve C shows the case of not force cooling the EGR gas.

When force cooling the EGR gas as shown by the curve A in FIG. 5, theamount of soot produced peaks when the EGR rate is a little under 50percent. In this case, if the EGR rate is made about 55 percent orhigher, almost no soot is produced any longer.

On the other hand, when the EGR gas is slightly cooled as shown by curveB in FIG. 5, the amount of soot produced peaks when the EGR rate isslightly higher than 50 percent. In this case, if the EGR rate is madeabove about 65 percent, almost no soot is produced any longer.

Further, when the EGR gas is not force cooled as shown by curve C inFIG. 5, the amount of soot produced peaks near an EGR rate near 55percent. In this case, if the EGR rate is made over about 70 percent,almost no soot is produced any longer. Note that FIG. 5 shows thehydrocarbons are exhausted from the combustion chamber 5 in the form ofa soot precursor or a state before that or exhausted from the combustionchamber 5 in the form of soot.

Now, to stop the growth of hydrocarbons in the state before theproduction of soot, it is necessary to suppress the temperature of thefuel and the gas around it at the time of combustion in the combustionchamber 5 to a temperature lower than the temperature where soot isproduced. In this case, it was learned that the heat absorbing action ofthe gas around the fuel at the time of combustion of the fuel has anextremely great effect in suppression of the temperature of the fuel andthe gas around it.

That is, if there is only air around the fuel, the vaporized fuel willimmediately react with the oxygen in the air and burn. In this case, thetemperature of the air away from the fuel does not rise that much. Onlythe temperature around the fuel becomes locally extremely high. That is,at this time, the air away from the fuel does not absorb the heat ofcombustion of the fuel much at all. In this case, since the combustiontemperature becomes extremely high locally, the unburned hydrocarbonsreceiving the heat of combustion produce soot.

On the other hand, when there is fuel in a mixed gas of a large amountof inert gas and a small amount of air, the situation is somewhatdifferent. In this case, the evaporated fuel disperses in thesurroundings and reacts with the oxygen mixed in the inert gas to burn.In this case, the heat of combustion is absorbed by the surroundinginert gas, so the combustion temperature no longer rises that much. Thatis, it becomes possible to keep the combustion temperature low. That is,the presence of inert gas plays an important role in the suppression ofthe combustion temperature. It is possible to keep the combustiontemperature low by the heat absorbing action of the inert gas. amount ofsmoke produced when the engine load is relatively high. When the engineload becomes small, the EGR rate at which the amount of soot producedpeaks falls somewhat and the lower limit of the EGR rate at which almostno soot is produced any longer falls somewhat. In this way, the lowerlimit of the EGR rate at which almost no soot is produced any longerchanges in accordance with the degree of cooling of the EGR gas or theengine load.

FIG. 6 shows the amount of mixed gas of EGR gas and air, the ratio ofair in the mixed gas, and the ratio of EGR gas in the mixed gas requiredfor making the temperature of the fuel and the gas around it at the timeof combustion a temperature lower than the temperature at which soot isproduced in the case of use of EGR gas as an inert gas and of delayingthe fuel injection timing. Note that in FIG. 6, the ordinate shows thetotal amount of suction gas taken into the combustion chamber 5. Thebroken line Y shows the total amount of suction gas able to be takeninto the combustion chamber 5 when supercharging is not being performed.Further, the abscissa shows the required load Z1 shows the operatingregion of a relatively low load.

Referring to FIG. 6, the ratio of air, that is, the amount of air in themixed gas, shows the amount of air necessary for causing the injectedfuel to completely burn. That is, in the case shown in FIG. 6, the ratioof the amount of air and the amount of injected fuel becomes thestoichiometric air fuel ratio. On the other hand, in FIG. 6, the ratioof EGR gas, that is, the amount of EGR gas in the mixed gas, shows theminimum amount of EGR gas required for making the temperature of thefuel and the gas around it a temperature lower than the temperature atwhich soot is produced when the injected fuel is burned. This amount ofEGR gas is, expressed in terms of the EGR rate, more than 55 percent—inthe embodiment shown in FIG. 6, more than 70 percent. That is, if thetotal amount of suction gas taken into the combustion chamber 5 is madethe solid line X in FIG. 6 and the ratio between the amount of air andamount of EGR gas in the total amount of suction gas X is made the ratioshown in FIG. 6, the temperature of the fuel and the gas around itbecomes a temperature lower than the temperature at which soot isproduced and therefore no soot at all is produced any longer. Further,the amount of NO_(x) produced at this time is around 10 ppm or less andtherefore the amount of NO_(x) produced becomes extremely small.

If the amount of fuel injected increases, the amount of heat generatedat the time of combustion of the fuel increases, so to maintain thetemperature of the fuel and the gas around it at a temperature lowerthan the temperature at which soot is produced, the amount of heatabsorbed by the EGR gas must be increased. Therefore, as shown in FIG.6, the amount of EGR gas has to be increased relative to the greater theamount of injected fuel. That is, the amount of EGR gas has to beincreased as the required load becomes higher.

On the other hand, in the load region Z2 of FIG. 6, the total amount ofsuction gas X required for inhibiting the production of soot exceeds thetotal amount of suction gas Y which can be taken in. Therefore, in thiscase, to supply the total amount of suction gas X required forinhibiting the production of soot, both the EGR gas and the suction gasbecome necessary or the EGR gas has to be supercharged or pressurized.When not supercharging or pressurizing the EGR gas etc., in the loadregion Z2, the total amount of suction gas X matches with the totalamount of suction gas Y able to be taken in. Therefore, in this case, toinhibit the production of soot, the amount of air is reduced somewhat toincrease the amount of EGR gas and the fuel is burned with a richair-fuel ratio.

As explained above, FIG. 6 shows the case of burning the fuel at thestoichiometric air-fuel ratio. Even if the amount of air in theoperating region Z1 shown in FIG. 6 is made smaller than the amount ofair shown in FIG. 6, however, that is, even if the air-fuel ratio ismade rich, it is possible to reduce the amount of production of NO_(x)to around 10 ppm or less while inhibiting the production of soot.Further, even if the amount of air in the operating region Z1 shown inFIG. 6 is made greater than the amount of air shown in FIG. 6, that is,even if the mean value of the air-fuel ratio is made lean, it ispossible to reduce the amount of production of NO_(x) to around 10 ppmor less while inhibiting the production of soot.

That is, if the air-fuel ratio is made rich, the fuel becomes excessive,but since the combustion temperature is suppressed to a low temperature,the excess fuel does not grow into soot and therefore no soot isproduced. Further, at this time, only an extremely small amount ofNO_(x) is produced. On the other hand, when the air-fuel ratio is leanor even when the air-fuel ratio is the stoichiometric air-fuel ratio, ifthe combustion temperature becomes high, a small amount of soot isproduced, but in the present invention, the combustion temperature issuppressed to a low temperature, so no soot at all is produced. Further,only an extremely small amount of NO_(x) is produced.

In this way, in the operating region Z1, regardless of the air-fuelratio, that is, whether the air-fuel ratio is rich, the stoichiometricair-fuel ratio, or lean, no soot is produced and the amount of NO_(x)produced becomes extremely small. Therefore, considering the improvementin the fuel consumption efficiency, it can be said to be preferable tomake the air-fuel ratio lean at this time.

If making the EGR rate more than 55 percent in this way, even if theair-fuel mixture is overly rich, no soot is produced any longer.Therefore, even when delaying the fuel injection timing, for example,even at a fuel injection timing of BTDC 20°, if the EGR rate is mademore than 55 percent, no soot is produced any longer. At this time, aswill be understood from FIG. 4, unburned hydrocarbons are produced, butthe amount of unburned hydrocarbons produced is considerably smallerthan the case of a fuel injection timing of BTDC 80° and therefore itbecomes possible to reduce the amount of fuel consumption compared withthe case of a fuel injection timing of BTDC 80°. Further, as will beunderstood from FIG. 4, if the EGR rate is increased until no soot isproduced any longer, the amount of NO_(x) produced becomes extremelysmall. Therefore, if the EGR rate is increased until an EGR rate whereno soot is produced any longer and the fuel injection timing is delayed,ideal combustion in which almost no soot and NO_(x) are produced and theamount of fuel consumption can be reduced is obtained.

Therefore, in the present invention, to perform this ideal combustion,the EGR rate is increased to an EGR rate where no soot is produced anylonger and the fuel injection timing is delayed. One of the advantagesof this new combustion is that there is no need to devise any measureswith regard to the formation of an overly rich air-fuel mixture,therefore there is great freedom in the structure of the combustionchamber, the arrangement of the fuel injectors and the spark plugs etc.,the injection timing, and the ignition timing. If performing this newcombustion, however, the EGR rate has to be made high, so the air-fuelmixture becomes difficult to ignite and accordingly sufficientconsideration is required to ensure stable ignition of the air-fuelmixture by the spark plug 7.

In the embodiment shown in FIG. 2, the fuel is injected from a fuelinjector 6 along the axial line of the cylinder in a conical shape. Iffuel is injected in a conical shape in this way, a sub spray flow F₂ isformed around the main spray flow F₁ forming the conical shape. In thisembodiment, the discharge gap of the spark plug 7 temperature at whichthe growth of the hydrocarbons stops midway to perform “firstcombustion”, that is, low temperature combustion, while when the engineload is relatively high, “second combustion”, that is, theconventionally performed combustion, is performed. Note that the firstcombustion, that is, the low temperature combustion, as clear from theexplanation up to here, means combustion where the amount of inert gasin the combustion chamber is larger than the amount of inert gas wherethe amount of production of the soot peaks and where almost no soot isproduced, while the second combustion, that is, the conventionallyperformed combustion, means combustion where the amount of inert gas inthe combustion chamber is smaller than the amount of inert gas where theamount of production of soot peaks.

FIG. 7 shows a first operating region I where the first combustion, thatis, the low temperature combustion, is performed and a second operatingregion II and third operating region III where the second combustion,that is, the combustion by the conventional combustion method, isperformed. Note that in FIG. 7, the ordinate L shows the amount ofdepression of the accelerator pedal 40, that is, the required load, andthe abscissa N shows the engine rotational speed. In the secondoperating region II, the fuel is injected twice, i.e., in the suctionstroke and the compression stroke, namely, two-stage injection isperformed, while in the third operating region III, the fuel is injectedduring the suction stroke, i.e., suction stroke injection is performed.These two-stage injection and suction stroke injection are injectionmethods used conventionally. Below, this combustion by two-stageinjection and combustion by suction stroke injection will be referred totogether as “second combustion”.

In FIG. 7, X(N) shows a first boundary between the first operatingregion I and the second operating region is arranged inside the subspray flow F₂ so as to prevent the ignition current from leaking due todeposition of carbon on the spark plug 7 and to ensure stable ignitionof the air-fuel mixture. When fuel is being injected from a fuelinjector 6, the air-fuel mixture of the sub spray flow F₂ is ignited bythe spark plug 7.

The main spray flow F₁ and the sub spray flow F₂ are formed stably atall times regardless of the operating state of the engine. Therefore, byarranging the discharge gap of the spark plug 7 in the sub spray flowF₂, it is possible to reliably ignite the air-fuel mixture at all times.Note that even right after the completion of the fuel injection, theair-fuel mixture gathers around the discharge gap of the spark plug 7,so it is also possible to ignite the air-fuel mixture by the spark plug7 right after completion of fuel injection.

If the injected fuel deposits on the inner wall surface of the cylinderbore, unburned hydrocarbons or smoke will be produced. Therefore, it ispreferable not to make the penetration force of the injected fuel toostrong so as to prevent the injected fuel from reaching the innercircumferential wall of the cylinder bore. Note that if the fuel isinjected along the axial line of the cylinder as shown in FIG. 2, itwill become harder for the injected fuel to reach the innercircumferential wall of the cylinder bore.

Note that the temperature of the fuel and its surrounding as at the timeof combustion in the combustion chamber can only be suppressed to notmore than a temperature where the growth of hydrocarbons stops midway atthe time of a relatively low engine load where the amount of heatgenerated by the combustion is relatively small. Therefore, in thisembodiment of the present invention, when the engine load is relativelylow, the fuel injection timing is delayed and the temperature of thefuel and its surrounding gas at the time of combustion is suppressed tonot more than a II, and Y(N) shows a second boundary between the firstoperating region I and the second operating region II. The change ofoperating regions from the first operating region I to the secondoperating region II is judged based on the first boundary X(N), whilethe change of operating regions from the second operating region II tothe first operating region I is judged based on the second boundaryY(N).

That is, when the engine operating state is in the first operatingregion I where the low temperature combustion is being performed, if therequired load L exceeds the first boundary X(N), which is a function ofthe engine rotational speed N, it is judged that the operating regionhas shifted to the second operating region II and second combustion isperformed. Next, when the required load L becomes lower than the secondboundary Y(N), which is a function of the engine rotational speed N, itis judged that the operating region has shifted to the first operatingregion I and the low temperature combustion is again performed.

Further, in FIG. 7, Z(N) shows a third boundary between the secondoperating region II and third operating region III.

Note that when the engine operating state is the first operating regionI where low temperature combustion is performed, almost no soot isproduced. Instead, the unburned hydrocarbons are exhausted from thecombustion chamber 5 as a soot precursor or a form before that. At thistime, the unburned hydrocarbons exhausted from the combustion chamber 5are oxidized well by the catalyst 20 having an oxidation function.

As the catalyst 20, an oxidation catalyst, three-way catalyst, or NO_(x)absorbent may be used. An NO_(x) absorbent has the function of absorbingNO_(x) when the air-fuel ratio in the combustion chamber 5 is lean andreleasing NO_(x) when the air-fuel ratio in the combustion chamber 5becomes the stoichiometric air-fuel ratio or rich.

The NO_(x) absorbent is for example comprised of alumina as a carrierand, on the carrier, for example, at least one of potassium K, sodiumNa, lithium Li, cesium Cs, and other alkali metals, barium Ba, calciumCa, and other alkali earths, lanthanum La, yttrium Y, and other rareearths plus platinum Pt or another precious metal.

An oxidation catalyst of course and also a three-way catalyst and NO_(x)absorbent have an oxidation function, therefore as explained above it ispossible to use a three-way catalyst or NO_(x) absorbent as the catalyst20.

Next, the control of the operation in the first operating region I, thesecond operating region II, and the third operating region III will beexplained in brief with reference to FIG. 8 and FIG. 9.

FIG. 8 shows the opening degree of the throttle valve 17, the openingdegree of the EGR control valve 24, the EGR rate, the air-fuel ratio,and the amount of injection with respect to the required torque L. Asshown in FIG. 8, in the first operating region I with the low requiredload L, the opening degree of the throttle valve 17 is graduallyincreased from the fully closed state to the half opened state as therequired load L becomes higher, while the opening degree of the EGRcontrol valve 24 is gradually increased from close to the fully closedstate to the fully opened state as the required load L becomes higher.Further, in the example shown in FIG. 8, in the first operating regionI, the EGR rate is made about 70 percent and the air-fuel ratio is madea lean air-fuel ratio. Note that in this example, the air-fuel ratio ismade leaner the smaller the required load L.

In other words, in the first operating region I, the opening degree ofthe throttle valve 17 and the opening degree of the EGR control valve 24are controlled so that the EGR rate becomes about 70 percent and theair-fuel ratio becomes a lean air-fuel ratio in accordance with therequired load L.

On the other hand, as shown in FIG. 9, in the first operating region I,the fuel injection Q₂ is performed between BTDC 25° to TDC in thecompression stroke. In this case, the injection start timing θS2 becomesearlier the higher the required load L. The injection end timing (θE2becomes later the higher the required load L.

Further, as shown in FIG. 9, the ignition timing timing θI is set tojust before the completion of the fuel injection. Therefore, in thisembodiment, when the fuel is being injected, the spark plug 7 isperforming its ignition action. At that time the air-fuel mixture of thesub spray flow F₂ (FIG. 2) is ignited by the spark plug 7 and the flameof ignition ignites the air-fuel mixture of the main spray flow F₁. Ifthe ignition action of the spark plug 7 is performed in this way duringthe fuel injection, the injected fuel will not sufficient disperse atthat time and the center part of the main spray flow F₁ will become aconsiderably overly rich air-fuel mixture. Therefore, at that time, theoverly rich air-fuel mixture will be burned by the flame of ignition,but almost no soot will be produced. Further, at this time, an extremelylean air-fuel ratio region will not be formed and therefore a largeamount of unburned hydrocarbons will not be produced either.

Note that as shown in FIG. 8, during idling operation, the throttlevalve 17 is made to close to close to the fully closed state. At thistime, the EGR control valve 24 is also made to close to close to thefully closed state. If the throttle valve 17 is closes to close to thefully closed state, the pressure in the combustion chamber 5 at thestart of compression will become low, so the compression pressure willbecome small. If the compression pressure becomes small, the amount ofcompression work by the piston 4 becomes small, so the vibration of theengine body 1 becomes smaller. That is, during idling operation, thethrottle valve 17 can be closed to close to the fully closed state tosuppress vibration in the engine body 1.

When the engine operating state is the first operating region I, almostno soot or NO_(x) is produced and the soot precursor or hydrocarbons ina form before that contained in the exhaust gas are oxidized by thecatalyst 20.

On the other hand, if the engine operating state changes from the firstoperating region I to the second operating region II, the opening degreeof the throttle valve 17 is increased in a step-like manner from thehalf opened state to the fully opened state. At this time, in theexample shown in FIG. 8, the EGR rate is reduced in a step-like mannerfrom about 70 percent to less than 40 percent and the air-fuel ratio isincreased in a step-like manner. That is, in the second operating regionII, as shown in FIG. 9, a first fuel injection Q₁ is performed at thestart of the suction stroke, while a second fuel injection Q₂ isperformed at the end of the compression stroke.

At this time, the first fuel injection Q₁ forms a uniform lean air-fuelmixture filling the combustion chamber 5 as a whole, while the air-fuelmixture formed by the second fuel injection Q₂ is ignited by the sparkplug 7. The flame of ignition becomes a source of ignition by which thelean air-fuel mixture filling the combustion chamber 5 is burned. Inthis way, the second fuel injection Q₂ is performed to form the sourceof ignition, so the amount of second fuel injection Q₂ is made asubstantially constant amount regardless of the required load.

In the second operating region II, the ignition action by the spark plug7 is performed immediately after the completion of the fuel injection.As explained above, immediately after the completion of the fuelinjection, the air-fuel mixture gathers around the spark plug 7,therefore the air-fuel mixture is reliably ignited.

When shifting from the first operating region I to the second operatingregion II, the EGR rate is made sharply smaller so as to jump over therange of the EGR rate (FIG. 5) where a large amount of smoke isproduced. At this time, if the fuel injection timing is left delayed asit is, there will be the risk of production of soot while the EGR rateis made sharply smaller to pass the range of EGR rate (FIG. 5) where alarge amount of smoke is produced. In this embodiment of the presentinvention, however, when shifting from the first operating region I tothe second operating region II, the injection timing of the majority ofthe fuel is made much earlier. That is, the majority of the fuel isinjected in the suction stroke. If the majority of the fuel is injectedin the suction stroke, soot is no longer produced regardless of the EGRrate. Therefore, there is no longer a risk of production of soot whilethe EGR rate is being made sharply smaller.

In the second operating region II, the opening degree of the throttlevalve 17 is made gradually larger the higher the required load L.Therefore, the EGR gradually falls and the air-fuel ratio becomesgradually smaller toward the stoichiometric air-fuel ratio the higherthe required load L.

Next, when the engine operating state changes from the second operatingregion II to the third operating region III, as shown in FIG. 8, thethrottle valve 17 is substantially held in the fully opened state.Further, to make the air-fuel ratio the stoichiometric air-fuel ratio,the opening degree of the EGR control valve 24 is made smaller thehigher the required load L. At this time, the air-fuel ratio iscontrolled by feedback to the stoichiometric air-fuel ratio based on theoutput signal of the air-fuel ratio sensor 22. After the EGR controlvalve 24 is fully closed, the air-fuel ratio is made richer the furtherhigher the required load L.

As shown in FIG. 9, in the third operating region III, fuel injection Q₁is performed at the start of the suction stroke. The injection starttiming θS1 and the injection end timing θE1 of the fuel injection Q₁performed at the start of the suction stroke, the injection start timingθS2 and injection end timing θE2 of the fuel injection Q₂ performed atthe end of the compression stroke, and the ignition timing θI arefunctions of the required load L and the engine rotational speed N. Theinjection start timing θS1 and the injection end timing θE1 of the fuelinjection Q₁, the injection start timing θS2 and injection end timingθE2 of the fuel injection Q₂, and the ignition timing θI are stored asfunctions of the required load L and the engine rotational speed in theROM 32 in advance in the form of maps.

FIG. 10 shows the air-fuel ratio A/F in the first operating region I. InFIG. 10, the curves shown by A/F=15, A/F=20, A/F=25, and A/F=30 showwhen the air-fuel ratio is 15, 20, 25, and 30, respectively. Theair-fuel ratios between the curves are determined by proportionaldistribution. As shown in FIG. 10, in the first operating region I, theair-fuel ratio becomes lean. Further, in the first operating region I,the air-fuel ratio A/F is made leaner the lower the required load L.

That is, the lower the required load L, the smaller the amount of heatgenerated by the combustion. Therefore, even if reducing the EGR ratethe lower the required load L, low temperature combustion becomespossible. If the EGR rate is reduced, the air-fuel ratio becomes largerand therefore, as shown in FIG. 10, the air-fuel ratio A/F is madelarger the lower the required load L. The larger the air-fuel ratio A/F,the more the fuel consumption efficiency is improved. Therefore, to makethe air-fuel ratio as lean as possible, in this embodiment of thepresent invention, the air-fuel ratio A/F is made larger the lower therequired load L.

Note that the target opening degree ST of the throttle valve 17 requiredfor making the air-fuel ratio the target air-fuel ratio shown in FIG. 10is stored in advance in the ROM 32 in the form of a map as a function ofthe required load L and the engine rotational speed N as shown in FIG.11A, while the target opening degree SE of the EGR control valve 24required for making the air-fuel ratio the target air-fuel ratio shownin FIG. 10 is stored in advance in the ROM 32 in the form of a map as afunction of the required load L and the engine rotational speed N asshown in FIG. 11B.

FIG. 12 shows the target air-fuel ratio when second-combustion, that is,ordinary combustion by the conventional method of combustion, isperformed. Note than in FIG. 12, the curves shown by A/F=14, A/F=14.6,A/F=15, and A/F=25 show the target air-fuel ratios 14, 14.6, 15, and 25.As will be understood from FIG. 12, in the second operating region IIbetween the first boundary X(N) and the third boundary Z(N), theair-fuel ratio A/F becomes leaner the lower the required load L.Further, in the low load side region of the third operating region IIIwith the larger required load L than the third boundary Z(N), theair-fuel ratio A/F is made 14.6, that is, the stoichiometric air-fuelratio.

Note that the target opening degree ST of the throttle valve 17 requiredfor making the air-fuel ratio the target air-fuel ratio shown in FIG. 12is stored in advance in the ROM 32 in the form of a map as a function ofthe required load L and the engine rotational speed N as shown in FIG.13A, while the target opening degree SE of the EGR control valve 24required for making the air-fuel ratio the target air-fuel ratio shownin FIG. 12 is stored in advance in the ROM 32 in the form of a map as afunction of the required load L and the engine rotational speed N asshown in FIG. 13B.

Next, the operational control will be explained with reference to FIG.14.

Referring to FIG. 14, first, at step 100, it is judged if a flag Ishowing that the engine operating region is the first operating region Iis set or not. When the flag I is set, that is, when the engineoperating region is the first operating region I, the routine proceedsto step 101, where it is judged if the required load L has become largerthan the first boundary X(N). When L≦X(N), the routine proceeds to step103, where low temperature combustion is performed.

That is, at step 103, the target opening degree ST of the throttle valve17 is calculated from the map shown in FIG. 11A and the opening degreeof the throttle valve 17 is made the target opening degree ST. Next, atstep 104, the target opening degree SE of the EGR control valve 24 iscalculated from the map shown in FIG. 11B and the opening degree of theEGR control valve 24 is made this target opening degree SE. Next, atstep 105, the injection start timing θS2 and injection end timing θE2 ofthe fuel injection Q₂ performed at the end of the compression stroke arecalculated from the map stored in the ROM 32 based on the required loadL and the engine rotational speed. The fuel injection is controlledbased on these. Next, at step 106, the ignition timing θI is calculatedfrom the map stored in the ROM 32 based on the required load L and theengine rotational speed and the ignition timing is controlled based onthis.

On the other hand, when it is judged at step 101 that L>X(N), theroutine proceeds to step 102, where the flag I is reset, then theroutine proceeds to step 109, where the second combustion is performed.

That is, at step 109, the target opening degree ST of the throttle valve17 is calculated from the map shown in FIG. 13A and the opening degreeof the throttle valve 17 is made the target opening degree ST. Next, atstep 110, the target opening degree SE of the EGR control valve 24 iscalculated from the map shown in FIG. 13B and the opening degree of theEGR control valve 24 is made this target opening degree SE. Next, atstep 111, it is judged if the required load L is higher than the thirdboundary Z(N) or not. When L≦Z(N), that is, when the engine operatingstate is the second operating region II, the routine proceeds to step112, where two-stage injection is performed.

That is, at step 112, the injection start timing θS1 and injection endtiming θE1 of the fuel injection Q₁ performed at the start of thesuction stroke and the injection start timing θS2 and injection endtiming θE2 of the fuel injection Q₂ performed at the end of thecompression stroke are calculated from the map stored in the ROM 32based on the required load L and the engine rotational speed. The fuelinjection is controlled based on these. Next, at step 113, the ignitiontiming θI is calculated from the map stored in the ROM 32 based on therequired load L and the engine rotational speed and the ignition timingis controlled based on this.

On the other hand, when it is judged at step 111 that L>Z(N), that is,when the engine operating state is the third operating region III, theroutine proceeds to step 114, where the normal uniform air-fuel mixturecombustion is performed.

That is, at step 114, the injection start timing θS1 and injection endtiming θE1 of the fuel injection Q₁ performed at the start of thesuction stroke are calculated from the map stored in the ROM 32 based onthe required load L and the engine rotational speed. The fuel injectionis controlled based on these. Next, at step 115, the ignition timing θIis calculated from the map stored in the ROM 32 based on the requiredload L and the engine rotational speed and the ignition timing iscontrolled based on this. Next, at step 116, when the target air-fuelratio is the stoichiometric air-fuel ratio, the opening degree of theEGR control valve 24 is controlled so that the air-fuel ratio becomesthe stoichiometric air-fuel ratio based on the-output signal of theair-fuel ratio sensor 22.

When the flag I is reset, at the next processing cycle, the routineproceeds from step 100 to step 107, where it is judged if the requiredload L has become lower than the second boundary Y(N) or not. WhenL≧Y(N), the routine proceeds to step 109, where the second combustion isperformed.

On the other hand, when it is judged at step 107 that L<Y(N), theroutine proceeds to step 108, where the flag I is set, then the routineproceeds to step 103, where low temperature combustion is performed.

Next, an explanation will be given of the case of using an NO_(x)absorbent as the catalyst 20.

If the ratio of the air and fuel (hydrocarbons) supplied into the engineintake passage, combustion chamber 5, and exhaust passage upstream ofthe NO_(x) absorbent 20 is referred to as the air fuel ratio of theexhaust gas flowing into the NO_(x) absorbent 20, then the NO_(x)absorbent 20 performs an NO_(x) absorption and release action in whichit absorbs NO_(x) when the air-fuel ratio of the inflowing exhaust gasis lean while releases the absorbed NO_(x) when the air-fuel ratio ofthe inflowing exhaust gas becomes the stoichiometric air-fuel ratio orrich. Note that when fuel (hydrocarbons) to air is not supplied to theexhaust passage upstream of the NO_(x) absorbent 20, the air-fuel ratioof the inflowing exhaust gas matches with the air-fuel ratio in thecombustion chamber 5. Therefore, in this case, as explained above, theNO_(x) absorbent 20 absorbs NO_(x) when the air-fuel ratio in thecombustion chamber 5 is lean while releases the absorbed NO_(x) when theair-fuel ratio in the combustion chamber 5 becomes the stoichiometricair-fuel ratio or rich.

If this NO_(x) absorbent 20 is placed in the engine exhaust passage, theNO_(x) absorbent 20 will in actuality perform an NO_(x) absorption andrelease action, but there are portions of the detailed mechanism of thisabsorption and release action which are still not clear. This absorptionand release action, however, is considered to be performed by themechanism shown in FIGS. 15A and 15B. Next, this mechanism will beexplained taking as an example the case of carrying platinum Pt andbarium Ba on the carrier, but the same mechanism applies even if usinganother precious metal and alkali metal, alkali earth, or rare earth.

That is, when the inflowing exhaust gas becomes lean, the concentrationof oxygen in the inflowing exhaust gas increases. At this time, as shownin FIG. 15A, the oxygen O₂ deposits on the surface of the platinum Pt inthe form of O₂ ⁻ or O²⁻. On the other hand, the NO in the inflowingexhaust gas reacts with the O₂ ⁻ or O²⁻ on the surface of the platinumPt to become NO₂ (2NO+O₂→2NO₂). Next, part of the produced NO₂ isoxidized on the platinum Pt and absorbed in the absorbent and diffusesinside the absorbent in the form of nitrate ions NO₃ ⁻ as shown in FIG.15A while bonding with the barium oxide BaO. The NO_(x) is absorbed inthe NO_(x) absorbent 20 in this way. So long as the concentration ofoxygen in the inflowing exhaust gas is high, NO₂ is produced on thesurface of the platinum Pt. So long as the NO_(x) absorption capabilityof the absorbent does not become saturated, the NO₂ is absorbed in theabsorbent and nitrate ions NO₃ ⁻ are produced.

On the other hand, when the air-fuel ratio of the inflowing exhaust gasis made rich, the concentration of oxygen in the inflowing exhaust gasfalls and as a result the amount of production of NO₂ on the surface ofthe platinum Pt falls. If the amount of production of NO₂ falls, thereaction proceeds in the reverse direction (NO₃ ⁻→NO₂) and therefore thenitrate ions NO₃ ⁻ in the absorbent are released from the absorbent inthe form of NO₂. At this time, the NO_(x) released from the NO_(x)absorbent 20 reacts with the large amount of unburnt hydrocarbons andcarbon monoxide contained in the inflowing exhaust gas to be reduced asshown in FIG. 15B. In this way, when there is no longer any NO₂ presenton the surface of the platinum PT, NO₂ is successively released from theabsorbent. Therefore, if the air-fuel ratio of the inflowing exhaust gasis made rich, the NO_(x) will be released from the NO_(x) absorbent 20in a short time and, further, the released NO_(x) will be reduced, so noNO_(x) will be discharged into the atmosphere.

Note that in this case, even if the air-fuel ratio of the inflowingexhaust gas is made the stoichiometric air-fuel ratio, NO_(x) will bereleased from the NO_(x) absorbent 20. When the air-fuel ratio of theinflowing exhaust gas is made the stoichiometric air-fuel ratio,however, the NO_(x) will be released from the NO_(x) absorbent 20 onlygradually, so a somewhat long time will be required for having all ofthe NO_(x) absorbed in the NO_(x) absorbent 20 be released.

There are, however, limits to the NO_(x) absorption capability of theNO_(x) absorbent 20. It is necessary to release the NO_(x) from theNO_(x) absorbent 20 before the NO_(x) absorption capability of theNO_(x) absorbent 20 becomes saturated. Therefore, it is necessary toestimate the amount of NO_(x) absorbed in the NO_(x) absorbent 20.Therefore, in this embodiment of the present invention, the amount ofNO_(x) absorption A per unit time when the first combustion is beingperformed is found in advance in the form of the map shown in FIG. 16Aas a function of the required load L and the engine rotational speed N,while the amount of NO_(x) absorption B per unit time when the secondcombustion is being performed is found in advance in the form of the mapshown in FIG. 16B as a function of the required load L and the enginerotational speed N_(x). The amount ΣNOX of NO_(x) absorbed in the NO_(x)absorbent 20 is estimated by cumulative addition of these amounts ofNO_(x) absorption A and B per unit time. Note that in this case theamount of NO_(x) absorption A is extremely small.

On the other hand, when the engine operating state is the thirdoperating region III, the air-fuel ratio is made the stoichiometricair-fuel ratio or rich. At this time, the NO_(x) is released from theNO_(x) absorbent 20. Therefore, in this embodiment of the presentinvention, the amount C of release of NO_(x) per unit time is calculatedfrom the map shown in FIG. 16C as a function of the required load L andthe engine rotational speed N, and the amount of NO_(x) release C issubtracted from the amount ΣNOX of the absorption of NO_(x) when tneair-fuel ratio is the stoichiometric air-fuel ratio or rich.

In this embodiment according to the present invention, when the amountΣNOX of NO_(x) absorption exceeds a predetermined maximum allowablevalue, the NO_(x) is made to be released from the NO_(x) absorbent 20.This will be explained next referring to FIG. 17.

FIG. 17 shows the processing routine of the NO_(x) releasing flag setwhen NO_(x) is to be released from the NOx absorbent 20. This routine isexecuted by interruption every predetermined time interval.

Referring to FIG. 17, first, at step 200, it is judged if a flag Ishowing that the engine operating region is the first operating region Iis set or not. When the flag I is set, that is, when the engineoperating region is the first operating region I, the routine proceedsto step 201, where the amount of absorption A of NO_(x) per unit time iscalculated from the map shown in FIG. 16A. Next, at step 202, A is addedto the amount ΣNOX of absorption of NO_(x). Next, at step 203, it isdetermined if the amount ΣNOX of absorption of NO_(x) has exceeded amaximum allowable value MAX. If ΣNOX>MAX, the routine proceeds to step204, where the NO_(x) releasing flag is set for a predetermined time.Next, at step 205, ΣNOX is made zero.

On the other hand, when it is determined at step 200 that the flag I hasbeen reset, the routine proceeds to step 206, where it is judged if therequired load L is higher than the third boundary Z(N). When L≦Z(N),that is, when the engine operating state is the second operating regionII, the routine proceeds to step 207, where the amount B of absorptionof NO_(x) per unit time is calculated from the map shown in FIG. 16B.Next, at step 208, B is added to the amount ΣNOX of the absorption ofNO_(x). Next, at step 209, it is determined if the amount ΣNOX of theabsorption of NO_(x) has exceeded the maximum allowable value MAX. WhenΣNOX>MAX, the routine proceeds to step 210, where the NO_(x) releasingflag 1 is set for a predetermined time, then ΣNOX is made zero at step211.

On the other hand, when it is judged at step 206 that L>Z(N), that is,when the engine operating state is the third operating region III, theroutine proceeds to step 212, where the amount C of release of NO_(x)per unit time is calculated from the map shown in FIG. 16C. Next, atstep 213, C is subtracted from the amount ΣNOX of the absorption ofNO_(x).

Next, an explanation will be made of the operation control referring toFIG. 18.

Referring to FIG. 18, first, at step 300, it is judged if a flag Ishowing that the engine operating region is the first operating region Iis set or not. When the flag I is set, that is, when the engineoperating region is the first operating region I, the routine proceedsto step 301, where it is judged if the required load L has become largerthan the first boundary X(N). When L≦X(N), the routine proceeds to step303, where low temperature combustion is performed.

That is, at step 303, the target opening degree ST of the throttle valve17 is calculated from the map shown in FIG. 11A and the opening degreeof the throttle valve 17 is made the target opening degree ST. Next, atstep 304, the target opening degree SE of the EGR control valve 24 iscalculated from the map shown in FIG. 11B and the opening degree of theEGR control valve 24 is made this target opening degree SE. Next, atstep 305, the ignition timing θI is calculated from the map stored inthe ROM 32 based on the required load L and the engine rotational speedand the ignition timing is controlled based on this.

Next, at step 306, it is judged if the NO_(x) releasing flag has beenset or not. When the NO_(x) releasing flag has not been set, the routineproceeds to step 307, where the injection start timing θS2 andinjection-end timing θE2 of the fuel injection Q₂ performed at the endof the compression stroke are calculated from the map stored in the ROM32 based on the required load L and the engine rotational speed. Thefuel injection is controlled based on these. At this time, lowtemperature combustion is performed under a lean air-fuel ratio.

As opposed to this, when it is judged at step 306 that the NO_(x)releasing flag has been set, the routine proceeds to step 308, where theinjection start timing θS2 and injection end timing θE2 of the fuelinjection Q₂ performed at the end of the compression stroke arecalculated from the map stored in the ROM 32 based on the required loadL and the engine rotational speed, then the amount of the fuel injectionQ₂ performed at the end of the compression stroke is increased to makethe air-fuel ratio rich by processing to make the injection start timingθS2 calculated from the map earlier. As a result, the air-fuel ratio ismade rich under the first combustion while the NO_(x) releasing flag isset.

On the other hand, when it is judged at step 301 that L>X(N), theroutine proceeds to step 302, where the flag I is reset, then theroutine proceeds to step 311, where the second combustion is performed.

That is, at step 311, the target opening degree ST of the throttle valve17 is calculated from the map shown in FIG. 13A and the opening degreeof the throttle valve 17 is made the target opening degree ST. Next, atstep 312, the target opening degree SE of the EGR control valve 24 iscalculated from the map shown in FIG. 13B and the opening degree of theEGR control valve 24 is made this target opening degree SE. Next, atstep 313, it is judged if the required load L is higher than the thirdboundary Z(N) or not. When L≦Z(N), that is, when the engine operatingstate is the second operating region II, the routine proceeds to step314, where two-stage injection is performed.

That is, first the ignition timing θI is calculated from the map storedin the ROM 32 based on the required load L and the engine rotationalspeed and the ignition timing is controlled based on this. Next, at step315, it is judged if the NO_(x) releasing flag has been set or not. Whenthe NO_(x) releasing flag has not been set, the routine proceeds to step316, where the injection start timing θS1 and the injection end timingθE1 of the fuel injection Q₁ performed at the start of the suctionstroke and the injection start timing θS2 and the injection end timingθE2 of the fuel injection Q₂ performed at the end of the compressionstroke are calculated from the map stored in the ROM 32 based on therequired load L and the engine rotational speed. The fuel injection iscontrolled based on these. At this time, two-stage injection isperformed under a lean air-fuel ratio.

On the other hand, when it is judged at step 315 that the NO_(x)releasing flag has been set, the routine proceeds to step 317, where theinjection start timing θS1 and injection end timing θE1 of the fuelinjection Q₁ performed at the start of the suction stroke and theinjection start timing θS2 and injection end timing θE2 of the fuelinjection Q₂ performed at the end of the compression stroke arecalculated from the map stored in the ROM 32 based on the required loadL and the engine rotational speed, then the amount of the fuel injectionQ₁ performed at the start of the suction stroke is increased to make theair-fuel ratio rich by processing to make the injection start timing θS1calculated from the map earlier. As a result, the air-fuel ratio is maderich under the second combustion while the NO_(x) releasing flag is set.

On the other hand, when it is judged at step 313 that L>Z(N), that is,when the engine operating state is the third operating region III, theroutine proceeds to step 318, where ordinary uniform air-fuel mixturecombustion is performed.

That is, at step 318, the injection start timing θS1 and the injectionend timing θE1 of the fuel injection Q₁ performed at the start of thesuction stroke are calculated from the map stored in the ROM 32 based onthe required load L and the engine rotational speed. The fuel injectionis controlled based on these. Next, at step 319, the ignition timing θIis calculated from the map stored in the ROM 32 based on the requiredload L and the engine rotational speed. Next, at step 320, when thetarget air-fuel ratio is the stoichiometric air-fuel ratio, the openingdegree of the EGR control valve 24 is controlled so that the air-fuelratio becomes the stoichiometric air-fuel ratio based on the outputsignal of the air-fuel ratio sensor 22.

When the flag I is reset, at the next processing cycle, the routineproceeds from step 300 to step 309, where it is judged if the requiredload L has become lower than the second boundary Y(N) or not. WhenL≧Y(N), the routine proceeds to step 311, where the second combustion isperformed.

On the other hand, when it is judged at step 309 that L<Y(N), theroutine proceeds to step 310, where the flag I is set, then the routineproceeds to step 303, where low temperature combustion is performed.

FIG. 19 shows an embodiment where the ignition action of the spark plug7 is performed immediately after the start of the fuel injection Q₂ atthe end of the compression stroke when the engine operating state is thefirst operating region I, that is, when low temperature combustion isbeing performed. That is, in this embodiment, the ignition timing θI isset to the same timing as the ignition timing θI of the embodiment shownin FIG. 9, while the injection start timing θS2 is set to immediatelybefore the ignition timing θI.

If setting the ignition timing θI in this way, the injected fuel isignited immediately after the start of injection and the flame ofignition causes the fuel injected after it to be successively burnedfairly much when being injected. Therefore, in this embodiment, no leanair-fuel mixture is formed, so there is no production of unburnedhydrocarbons due to combustion of a lean air-fuel mixture. Therefore, inthis embodiment as well, it is possible to prevent the production ofsoot and NO_(x) and reduce the amount of fuel consumption.

FIG. 20 shows another embodiment of the internal combustion engine.

In this embodiment, the spark plug 7 is arranged at the center of theinner wall surface of the cylinder head 3, the fuel injector 6 isarranged near the inner circumferential wall of the cylinder head 3, anda semispherically shaped groove 4 a extending from below the fuelinjector 6 to below the spark plug 7 is formed in the top surface of thepiston 4. When the engine operating state is the first operating regionI, that is, when low temperature combustion is being performed, the fuelis injected from the fuel injector 6 at the end of the compressionstroke by a small angle of spray toward the inside of the groove 4 a soas to follow the bottom surface of the groove 4 a. This injected fuel Fis guided by the bottom surface of the groove 4 a and is raised frombelow the spark plug 7 toward the spark plug 7.

In this embodiment, it takes time for the injected fuel to reach aroundthe spark plug 7, so the ignition action of the spark plug 7 is.performed after the completion of the injection. Therefore, as shown inFIG. 21, in the first operating region I, the ignition timing θI becomeslater than the injection end timing θE2 of the fuel injection Q₂.

In this embodiment, the injected fuel has to reach around the spark plug7 regardless of the amount of fuel injection and the injected fuel Fmust not disperse too much before the ignition action of the spark plug7. Therefore, in this embodiment, it is preferable to inject thespray-of fuel with a small angle of spray from the fuel injector 6 andwith a large penetration force.

As explained above, according to the present invention, it is possibleto prevent the production of soot and NO_(x) and to reduce the amount offuel consumption.

What is claimed is:
 1. An internal combustion engine provided with a spark plug for igniting fuel injected into a combustion chamber, wherein an amount of soot produced peaks when an amount of inert gas in the combustion chamber is increased and fuel injection timing in a compression stroke is delayed, and wherein, switching means is provided for selectively switching between a first combustion where substantially no soot is produced as the amount of inert gas in the combustion chamber exceeds the amount of inert gas at which the amount of soot produced peaks and a second combustion where the amount of inert gas in the combustion chamber is smaller than the amount of inert gas at which the production of soot peaks.
 2. An internal combustion engine as set forth in claim 1, wherein the fuel injection timing is set to a timing where a peak of an amount of production of soot appears when the amount of inert gas is increased.
 3. An internal combustion engine as set forth in claim 2, wherein the fuel injection timing is set to the end of the compression stroke.
 4. An internal combustion engine as set forth in claim 3, wherein an ignition action of the spark plug is performed during the fuel injection.
 5. An internal combustion engine as set forth in claim 3, wherein an ignition action of the spark plug is performed after the completion of the fuel injection.
 6. An internal combustion engine as set forth in claim 1, wherein the spark plug and the fuel injector are arranged adjoining each other at a center of an inner wall of a cylinder head and wherein a discharge gap of the spark plug is arranged in a sub spray flow formed around a main spray flow of the fuel injector.
 7. An internal combustion engine as set forth in claim 6, wherein fuel is injected from the fuel injector along an axis of the cylinder.
 8. An internal combustion engine as set forth in claim 1, wherein the spark plug is arranged at the center of an inner wall surface of a cylinder head, a fuel injector is arranged in a peripheral portion of an inner wall surface of the cylinder head, a groove extending from below the fuel injector to below the spark plug is formed in a top surface of a piston, and the fuel injected from the fuel injector to the inside of the groove is guided by a bottom surface of the groove to be directed around the spark plug.
 9. An internal combustion engine as set forth in claim 1, wherein an exhaust gas recirculation device is provided for recirculating exhaust gas exhausted from the combustion chamber into an intake passage of the engine and the inert gas comprises recirculated exhaust gas recirculated in the engine intake passage.
 10. An internal combustion engine as set forth in claim 9, wherein an exhaust gas recirculation rate is more than about 55 percent.
 11. An internal combustion engine as set forth in claim 9, wherein the exhaust gas recirculation device is provided with a cooler for cooling the recirculated exhaust gas.
 12. An internal combustion engine as set forth in claim 1, wherein unburned hydrocarbons are exhausted from the combustion chamber in the form of a soot precursor or a state before that rather than in the form of soot and wherein an after-treatment device for oxidizing the unburned hydrocarbons exhausted from the combustion chamber is arranged in an engine exhaust passage.
 13. An internal combustion engine as set forth in claim 12, wherein said after-treatment device is comprised of a catalyst having an oxidation function.
 14. An internal combustion engine as set forth in claim 13, wherein said catalyst is comprised of at least one of an oxidation catalyst, a three-way catalyst, and an NO_(x) absorbent.
 15. An internal combustion engine as set forth in claim 1, wherein an air-fuel ratio in the combustion chamber is made the stoichiometric air-fuel ratio or a lean air-fuel ratio or a rich air-fuel ratio.
 16. An internal combustion engine as set forth in claim 1, wherein the operating regions of the engine are divided into a low load side operating region and a high load side operating region, wherein the first combustion is performed in the low load side operating region, and the second combustion is performed in the high load side operating region.
 17. An internal combustion engine as set forth in claim 16, wherein in a low load side region of the high load side operating region, fuel is injected twice in a suction stroke and an end of a compression stroke.
 18. An internal combustion engine as set forth in claim 1, wherein an NO_(x) absorbent which absorbs NO_(x) contained in an exhaust gas when an air-fuel ratio of an inflowing exhaust gas is lean and releases the absorbed NO_(x) when the air-fuel ratio of the inflowing exhaust gas becomes the stoichiometric air-fuel ratio or rich is arranged in the engine exhaust passage and wherein the air-fuel ratio in the combustion chamber is made the stoichiometric air-fuel ratio or rich when NO_(x) should be released from the NO_(x) absorbent.
 19. An internal combustion engine as set forth in claim 16, wherein the high load side operating region includes a third combustion that releases NO_(x) from an NO_(x) absorbent when the air-fuel ratio in the combustion chamber is stoichiometric or rich. 